1. Field of the Invention
The present invention relates generally to a furnace for heating the interior space of a building structure through the combustion of solid materials such as wood or coal.
2. Description of the Related Art
Improved wood burning stoves and furnaces have been proposed in prior U.S. patents. In an effort to improve efficiency in extracting heat from the burning fuel, these devices typically include a firebox surrounded by an enclosing cabinet through which air may be circulated to extract heat.
U.S. Pat. No. 4,047,515, issued Sep. 13, 1977 to Walter L. Daniel, modifies such a device by providing a blower for forcing air into the interior of the firebox through louvers located on the opposite side walls of the firebox at a level adjacent the fire. Although a significant improvement over the prior art, the delivery of air to the fire at widely spaced apart points is less than optimum. Further, the continued use of large juxtaposed compartments for extracting heat from the fire fails to derive as much heat as possible from the fire.
U.S. Pat. No. 4,404,954 issued Sep. 20, 1983 to Steel discloses a fireplace furnace comprised of: an enclosed primary combustion chamber and a primary air supply for the initial burning of fuel; a separate enclosed secondary combustion chamber and a secondary air supply for burning any volatile gases remaining from the combustion in the primary chamber; and an enclosed room air heating chamber. The secondary combustion chamber is located between the primary chamber and the room air chamber. The flow of the air of the room air chamber is counter to the flow of flue gas in the secondary chamber, so that the heat transferred to the room air is from the flue gas of the secondary combustion chamber and none of the heat transferred to the room air is transferred directly from the primary combustion chamber.
U.S. Pat. No. 4,596,231, issued Jun. 24, 1986 to Chelminski shows a self-cleaning high heat exchange wood or coal stove which includes an interior cylindrical squirrel-cage configured fuel grate comprised of parallel air-conducting pipes for heating cold air forced by a blower. This stove relies on the cyclical condensation and combustion of effluent deposits as the interior cylindrical fuel grate is rotated by the user turning a handle.
U.S. Pat. No. 4,347,831 issued Sep. 7, 1982 to Graziano describes a fuel burning stove having a stove housing and a combustion chamber. The flue has a flue passage back to a lower portion of the combustion chamber so that unburned gaseous products of combustion rise toward the top of the chamber and are trapped and forced back downward through the chamber to be passed through the flame of combustion another time with the purposes of promoting efficient combustion and a cleaner combustion residue.
U.S. Pat. No. 4,738,241 issued Apr. 19, 1988 to Bernelov et al. provides a woodburning stove whose efficiency is increased by: preheating inlet air before it enters heat exchange tubes in the firebox; discharging hot air from a jacketed airspace along with hot air from the heat exchange tubes; deflecting discharged hot air downward; deflecting fire and hot gases from the firebox away from the flue so they must transfer heat to the top of the stove before they enter the flue; and, lastly, providing a second grate below the heat exchange tubes in order to catch falling burning embers.
The efficient combustion of flammable materials in wood burning furnaces has long been a cause for concern. Many of these devices have a tendency to develop extremely hot fires, consuming excessive amounts of wood and causing furnace components to warp or crack under the high temperatures which are developed. On the other hand, other furnace models incompletely combust the materials placed therein. The resulting inefficiency of the process of combustion thus increases the amount of fuel required to adequately heat a given space and increases the amount of pollutants released into the environment.
These efficiency problems have been attributed principally to the manner in which air for combustion is supplied to the fuel disposed in the firebox of the furnace. Usually, air is introduced beneath a frame on which the fuel is supported and flows upwardly through the burning materials. In certain instances, the rate of air flow to the fuel is greater than that required for optimum burning, causing an unnecessarily hot fire to develop and large amounts of fuel to be wasted. On the other hand, if air delivery rates are too low, the fuel may not burn completely. Thus, a need exists for a furnace which is able to efficiently combust fuel with a relatively low temperature fire as well as efficiently extract the resultant heat for warming the interior of a building structure.